var vm = new Vue({
    el:'#app',
    data: {
        km: 0,
        m: 0
    },
    methods: {
        kmtom:function(val) {
            this.m = this.km * 1000;
        },
        mtokm:function(val) {
            this.km = this.m / 1000;
        }
    },
    watch: {
        km:'kmtom',
        m:'mtokm'
    }
})